In vivo MR spectroscopy (MRS) provides considerable information about biochemical processes and metabolic status relevant for biological research and clinical diagnosis. However, since metabolism can vary greatly between different tissue types, it is essential to minimize partial volume effects in the observed NMR signal. This is typically achieved through the use of well-defined spatial localization.
Two groups of spatial localization techniques have been extensively developed for in vivo MRS applications: single-voxel localization and spectroscopic imaging (MRSI). Single-voxel localization techniques use frequency selective RF pulses in combination with constant amplitude gradients to define a small rectangular cube as a region-of-interest (ROI). Popular methods include image-selected in vivo spectroscopy (ISIS) (1), stimulated echo acquisition mode (STEAM) (2), and point-resolved spectroscopy (PRESS) (3). MRSI techniques (4) use phase encoding gradients to encode spatial information.
Signal sensitivity and spatial specificity are two competing factors in both groups of spatial localization techniques. While the localized volume would ideally be large to achieve high signal-to-noise ratio (SNR) at high temporal resolution, the cubic size would typically lead to inclusion of several different tissue types. This so-called partial volume effect complicates interpretation of spectroscopic information, as tissues with different metabolic profiles and status contribute to the signal. This is especially true when dealing with diseased tissues with diffuse abnormalities, as for example encountered in epilepsy (5), but even differentiating metabolism in cerebral or cerebellar gray and white matter is complicated by partial volume effects inherent to conventional (cubic) localization methods. Averaging NROI MRSI pixels over a given ROI will lead to a (NROI)1/2 penalty in SNR per unit of time compared to a spectrum directly acquired from the ROI.
Therefore, a heretofore unaddressed need still exists in the art to address the aforementioned deficiencies and inadequacies.